Ride with a slower person
Ride with somebody who also wants a slow ride and make sure you can talk easily.
Take a camera, ride in a scenic area, and promise yourself you'll take pictures along the ride.
Ride in urban areas with stop signs each block.
I too have the problem of starting a ride intending to take it easy. I'm not wired to ride easy if I have energy to go faster.
On days I am lazy but need to ride, I tell myself to just get on the bike for a short easy spin. Once I'm on the bike I end up pushing myself anyhow.